There are two basic types of panels:
Flat panel: Liquid (distilled water or a water-glycol mix) flows through tubes in the panel and is heated directly by the sun.  The panel glass and backing are specially designed to absorb heat and prevent that heat from being radiated back out of the panel.


Evacuated tube panels:  The panels have a set of long glass tubes with metal absorber tubes in their cores, which are heated by the sun.  Liquid runs past fins attached to the tops of the absorber tubes and is heated.  Because there is a vacuum in the tubes (no air), they are very efficient heat collectors.  The round tubes also help to capture the full strength of the sun, even when the sun is not pointing directly at the panel. The tubes are backed by a heat-absorbing backing to maximize solar heating.

In both cases, the glass used is very strong and won’t break under normal weather conditions or use.

And there are two basic types of systems:
Seasonal: These systems can only be operated when temperatures are sure to remain above freezing (e.g., late spring, summer and early fall).  They take advantage of the prime solar months while reducing upfront costs, but won’t deliver year-round energy savings.

Four-season: These systems use special fluids (e.g., food grade glycol) or a self-draining system to avoid freezing in winter.  They are more expensive, but deliver solar heat year round, even on the coldest winter days, as long as the sun is shining.

Backup Heating: Solar water heating systems almost always require a backup system for cloudy days and when your demand for hot water is higher than normal. Conventional storage water heaters usually provide backup and may already be part of the solar system package, but you may also want to consider switching to an energy-efficient on demand (tankless or instantaneous) water heater for backup. These units are essentially a box on your wall that quickly heat energy on demand, and reduce the amount on energy lost from traditional storage tanks.

More information
Natural Resources Canada has some excellent information on choosing a solar water system:
An Introduction to Solar Water Heating, www.canren.gc.ca/prod_serv/index.asp?CaId=141&PgId=750 (also available as pdf)

Solar Water Heating Systems: A Buyer's Guide, www.canren.gc.ca/prod_serv/index.asp?CaId=138&PgId=744  (also available as pdf) See especially the sections:
“What type of solar water heater should we buy?”
“What do we need to know before we contact a dealer?”
“How do we select a dealer?”

The Canadian Solar Industries Association (CANsia) offers a good in-depth guide to assessing your home’s suitability for solar Making Your Home Ready For Solar Water Heating, www.cansia.ca/downloads/solarready/GuideEnglish.pdf .

A good quick overview can be found at TriplePundit blog, “Four Things to Consider Before Going Solar,” www.triplepundit.com/pages/4-things-to-consider-before-go-002981.php

The Clean Air Foundation's Go Solar website is another great resource for information on solar technologies (hot water and photovoltaic), incentive programs and solar information sessions and workshops. Visit www.gosolarontario.ca for all the latest on solar energy.

In this example of a drainback system, distilled water heated by the solar collectors is fed directly into the home's hot water system. The existing hot water heater acts as a temperature booster, if necessary, for the water stored in the solar tank.

Some systems use glycol (an antifreeze) to prevent the system from freezing in winter. The glycol is circulated through a heat exchanger where the warmth from the glycol is passed through to the water in the solar storage tank. The glycol and your home's water supply never mix. And once again, the existing hot water heater acts as a temperature booster, if necessary, for the water stored in the solar tank.